1. Technical Field
The present invention is directed to a lithographic process for device fabrication in which charged particle energy is used to delineate a pattern in an energy sensitive material. The pattern is delineated by projecting the charged particle energy onto a patterned mask, thereby projecting an image of the mask onto the energy sensitive material.
2. Art Background
In device processing, an energy sensitive material, denominated a resist, is coated on a substrate such as a semiconductor wafer (e.g., a silicon wafer), a ferroelectric wafer, an insulating wafer, (e.g. a sapphire wafer), a chromium layer supported by a substrate, or a substrate having a combination of such materials. An image of a pattern is introduced into the resist by subjecting the resist to patterned radiation. The image is then developed to produce a patterned resist using expedients such as a solution-based developer or a plasma etch to remove one of either the exposed portion or the unexposed portion of the resist. The developed pattern is then used in subsequent processing, e.g. a mask to process, i.e. etch, the underlying layer. The resist is then removed. For many devices, subsequent layers are formed and the process is repeated to form overlying patterns in the device.
In recent years, lithographic processes in which a charged particle beam is used to delineate a pattern in an energy sensitive resist material have been developed Such processes provide high resolution and high throughput. One such process is the SCALPEL(copyright) (scattering with angular limitation projection electron beam lithography) process. The SCALPEL(copyright) process is described in U.S. Pat. No. 5,260,151 which is hereby incorporated by reference.
Referring to FIG. 1, a doublet lens system 15 is used in the lithography tool for the SCALPEL(copyright) process. A first lens system (not shown) is used to direct and focus incident radiation 10 from the radiation source (not shown) onto the mask 20. The mask 20 is used to pattern particle beam 10. The entire mask 20 is not illuminated at once. Mask 20, as shown, consists of a membrane 13, which is transparent to the particle beams incident thereon, and blocking regions 14.
The developed image of the mask pattern is defined by blocking regions 14, which scatter the particle beams 10 incident thereon. Unblocked illumination, illustrated as beams 12, is transmitted through the membrane regions 13. Blocked illumination, illustrated as beams 11 is caused to converge by means of a first electromagnetic/electrostatic projector lens 30 in lens system 15. Filter 19 is an aperture scatter filter. The aperture scatter filter 19 is designed so that the unscattered radiation (beams 12) passes through the aperture 21 therein. The scattered radiation 11 is blocked by the aperture scatter filter 19, which is located in the mutual focal plane of the lenses 30 and 31.
Second projector lens 31 of lens system 15 is of such configuration and so powered as to bring the unscattered beams 12 into an approximately parallel relationship. The action of the lens 31 is sufficient to direct beams 12 into orthogonal incidence onto wafer 24.
Lens system 15 consists of two lenses. Consequently, the lens system is referred to as a doublet electromagnetic lens arrangement. Such a doublet electromagnetic lens arrangement is described in Waskiewicz, W., et al., xe2x80x9cElectron-optics method for High-Throughput in a SCALPEL system: preliminary analysis,xe2x80x9d Microelectronic Engineering, Vol. 41/42, pp. 215-218 (1998). The doublet electromagnetic lens system described in Waskiewicz et al. provides telecentric reduction imaging from the mask to the wafer. Such an arrangement uses two lenses of similar construction. The lenses are laid out sequentially and separated by a distance equal to the sum of their two focal lengths. Referring again to FIG. 1, the object (i.e. the mask 20) is located in the back focal plane of the first projector lens 30 of lens system 15. An image of the mask 20 is formed at the front focal plane (i.e. the layer of energy sensitive material 23 on wafer 24) of the second projector lens 31 of second lens system 15. The magnification provided by the lens system is determined by the ratio of the focal length of lens 30 to the focal length of lens 31. The bore (D) to gap (S) ratio for both lenses are identical and the excitations (NI) are set equal but opposite.
When designed properly, the doublet lens not only substantially eliminates the rotation introduced into the image by an individual lens in the doublet, but also eliminates rotation-related aberrations in the image. These aberrations are primarily chromatic aberrations. Removing these aberrations provides the lowest total image blur. Doublet lens systems are described in Heritage, M. B., xe2x80x9cElectron-projection microfabrication system,xe2x80x9d J. Vac. Sci. Technol., Vol. 12, No. 6, pp. 1135-1140 (1975), which is hereby incorporated by reference.
In the classic magnetic doublet design, the first and second lenses are separated along their common optical axis to ensure that there is a space between the lenses that is field-free. The field-free space is a space that is not affected by the magnetic field generated by the lenses. Typically, both lenses have a common focal length (F) within this field-free space. Such an arrangement is illustrated in FIG. 2. FIG. 2 illustrates the magnetic flux as a function of distance along the optical axis relative to the position of magnetic lenses 30 and 31. In the region between lenses 30 and 31 the magnetic flux is zero. This is the desired field-free space.
However, in certain applications, design constraints do not permit the spacing between the first and second lenses that provides for a field-free space. In the lithography tool for the SCALPEL(copyright) process, for example, the mutual focal plane of lenses 30 and 31 is at the apertured scatter filter 19. Furthermore, in order to increase the speed at which the image is written (and thereby to achieve the desired throughput from the tool) the electron beam scans about the optical axis. In order to control the off-axis aberrations, e.g. astigmatism, that result from off-axis scanning, the bore of the doublet lens is increased while the axial separation between the two lenses either remains the same or is shortened to control space-charge blur. Consequently, the magnetic fields of the doublet lens overlap. This problem is illustrated in FIG. 3. In FIG. 3, the magnetic flux of each lens is affected by this overlap. This is observed with reference to dashed line 50 in FIG. 3. Observe that, due to the proximity between lenses 30 and 31, the flux as a function of axial position for lens 31 on one side of line 50 is not a mirror image of flux as a function of axial position on the other side of line 50. Thus, the desired axial magnetic field symmetry for lens 31 (and for lens 30) in FIG. 3 is not preserved.
This overlap causes field distortion. Also, the apertured scatter filter 19 is immersed in the magnetic field of lenses 30 and 31. Since this field overlap compounds aberrations and total blur growth and also causes projection magnification changes. Consequently, a solution to the magnetic field overlap of the projection lens doublet that is compatible with the SCALPEL(copyright) tool design is sought.
The present invention is directed to a magnetic doublet lens system in which the spacing between the two lenses is such that their magnetic fields overlap. The magnetic doublet lens system is equipped with magnetic clamps that effect substantial separation of the magnetic fields. The magnetic clamps are made of a ferromagnetic material. The present invention is also directed to an apparatus for electron beam lithography that has a magnetic doublet lens system that is equipped with magnetic clamps to effect substantial separation of the magnetic fields between the two lenses. Substantial separation, in the context of the present invention, is sufficient separation of ensure that the magnetic field of one lens in the doublet lens system is not adversely affected by the magnetic field of the other lens in the magnetic doublet lens system. Adverse affects are doublet compound image aberrations, total blur growth and projection magnification changes attributable to magnetic field overlap. However, one skilled in the art will appreciate that the magnetic clamps have a configuration and placement that preserves the symmetry of the magnetic doublet lens and the common focal plane of the two lenses in the magnetic doublet lens system. For example, if the magnetic doublet lens is designed for 4:1 image demagnification (i.e., image reduction), then the magnetic clamps are designed to preserve this relationship. An example of a suitable clamp design for such a magnetic doublet lens is one in which the dimensional relationship (i.e. for cylindrical clamps the ratio of height and diameter) is also 4:1.